Method for forming conversion coating on outer surface of magnesium or magnesium alloy

ABSTRACT

Disclosed is a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy by surface treatment with acidic and alkaline solutions. The method is carried out in an environmentally friendly manner to improve the corrosion resistance of the magnesium or magnesium alloy. The method minimizes the damage to the appearance of the magnesium or magnesium even after surface treatment and maximizes the attractive appearance of the magnesium or magnesium alloy.

BACKGROUND

1. Technical Field

The present invention relates to a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy. More specifically, the present invention relates to a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy in an environmentally friendly manner to improve the corrosion resistance of the magnesium or magnesium alloy.

2. Description of the Related Art

Magnesium is one of the most abundant elements found on the earth. Magnesium is the most lightweight structural metal material having a density of about 1.74 g/cm³. Based on these advantages, magnesium is currently used in a variety of applications, including aircraft, automobiles and portable device, and its usages and application are increasingly extended.

Magnesium alloys, however, have the drawback of poor corrosion resistance on account of their material properties. For better corrosion resistance, magnesium alloys should undergo chemical surface treatment. A series of conversion coating and finish coating steps is one of the most popular chemical treatment processes.

Conversion coating is a metal surface treatment process in which a coating is formed to a thickness of hundreds of nm on the surface of magnesium by various chemical reactions. The kind of the coating varies depending on a conversion coating solution used. Unlike electroplating wherein external electricity is applied, conversion coating is characterized in that a coating is formed on the surface of a metal by chemical reactions without the need to apply external electricity.

Such conversion coating processes of magnesium alloys are typically divided into chromate conversion coating and non-chromate conversion coating processes.

The first chromate conversion coating process, termed Dow1 or Dow21, were developed by Dow Chemical in the 1970s. According to the representative Dow's process, the surface of a metal is treated in an acidic solution containing sodium bichromate as a major component. The acidic solution is easy to handle and manage and the surface-treated metal has the advantages of good corrosion resistance and coating adhesion. However, the presence of hexavalent chromium, which was found to be toxic, in the acidic solution limits the use of the Dow processes.

Under these circumstances, low-chromate and non-chromate conversion coating processes have been developed. Non-chromate conversion coating processes are known to use various metal salts, such as manganese and zirconium salts, rare earth salts and organic salts. However, coating solutions used in the non-chromate conversion coating processes are highly priced and are difficult to handle.

Chromate conversion coatings are green or reddish grey and non-chromate conversion coatings are grey or ivory in color. That is, chromate conversion coatings do not sufficiently express the inherent color of magnesium, impairing the attractive appearance of magnesium alloys.

BRIEF SUMMARY

An object of the present invention is to provide a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy to improve the corrosion resistance of the magnesium or magnesium alloy.

Another object of the present invention is to provide a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy in an environmentally friendly manner.

Another object of the present invention is to provide a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy to minimize the damage to the appearance of the magnesium or magnesium even after surface treatment.

Still another object of the present invention is to provide a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy to maximize the attractive appearance of the magnesium or magnesium alloy.

According to an embodiment of the present invention, there is provided a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy, the method comprising: degreasing the outer surface of magnesium or a magnesium alloy, followed by water washing; etching the degreased magnesium or magnesium alloy to remove an oxide coating present on the outer surface thereof, followed by water washing; removing smut present on the outer surface of the magnesium or magnesium alloy (desmutting), followed by water washing; forming a conversion coating on the desmutted outer surface of the magnesium or magnesium alloy, followed by water washing; and forming a coating on the conversion coating.

According to the method of the present invention, the corrosion resistance of magnesium or a magnesium alloy is improved, the damage to the appearance of magnesium or a magnesium alloy is minimized even after surface treatment, and the attractive appearance of magnesium or a magnesium alloy can be maximized.

In the step of degreasing and water washing, the degreasing may be performed using a first aqueous solution containing 30-60 g/L of potassium hydroxide (KOH), 150-200 g/L of sodium hydroxide (NaOH), 20-30 g/L of an amine oxide, 5-10 g/L of sodium gluconate (C₆H₁₁NaO₇) and a wetting agent. The first aqueous solution can facilitate the removal of oily components from the surface of the magnesium or magnesium alloy and can minimize the discoloration of the metal material.

In the step of etching and water washing, the etching may be performed using a second aqueous solution containing 600-800 g/L of nitric acid (HNO₃), 200-400 g/L of phosphoric acid (H₃PO₄) and a wetting agent. The second aqueous solution may be diluted with distilled water to a pH of 1.0-2.0. The second aqueous solution may be in the temperature range of about 35 to about 50° C. The second aqueous solution can facilitate the removal of an oxide coating present on the surface of the magnesium or magnesium alloy and can minimize the discoloration of the metal.

In the step of desmutting and water washing, the desmutting may be performed by immersing the etched metal in a third aqueous solution containing 50-200 g/L of potassium hydroxide (KOH) and 5-10 g/L of a wetting agent. Within this composition, the third aqueous solution can prevent the formation of a hydroxide coating on the surface of the metal while preventing the metal from being desmutted.

In the step of forming a conversion coating and water washing, the conversion coating may be formed using a fourth aqueous solution containing 600-800 g/L of nitric acid (HNO₃), 200-400 g/L of phosphoric acid (H₃PO₄) and 0.5-10 g/L of hydrofluoric acid (HF). The fourth aqueous solution may be diluted with distilled water to a pH of 2.0-4.0. The conversion coating may be formed in the fourth aqueous solution at 20-60° C. Within the limited temperature range of the fourth aqueous solution, the conversion coating can be formed to a constant thickness at a controlled rate while minimizing the damage to the appearance of the metal.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1 is a magnification image showing the surface texture of a magnesium alloy plate whose surface is treated by a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy according to an embodiment of the present invention; and

FIG. 2 is a flow chart illustrating a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy according to an embodiment of the present invention.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, these embodiments are not intended to limit the scope of the invention. The same reference numerals denote the same parts throughout the drawings.

FIG. 1 is a magnification image showing a surface texture of a magnesium alloy plate whose surface is treated by a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy according to an embodiment of the present invention, and FIG. 2 is a flow chart illustrating a method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy according to an embodiment of the present invention.

It is to be understood that although an explanation is given of a method for the formation of a conversion coating on the surface of a magnesium alloy in order to explain the drawings, a conversion coating can be formed on the surface of a magnesium plate according to conditions required in the invention.

Referring to FIGS. 1 and 2 and Table 1, a first aqueous solution capable of degreasing the surface of a molded magnesium alloy is prepared to remove oils, fats and impurities attached to the molded metal surface and to prevent the discoloration of the metal.

TABLE 1 Composition Amine Sodium Treatment Treatment KOH NaOH oxide gluconate Additive Temp. time 30-60 g/L 150-200 g/L 20-30 g/L 5-10 g/L 3-5 g/L 35-50° C. 0.5-2 min

The additive contained in the first aqueous solution is a wetting agent. The wetting agent may be a surfactant for allowing the first aqueous solution to permeate the surface of the metal. Various wetting agents may be used depending on conditions required in the invention.

If the first aqueous solution is out of the composition indicated in Table 1, the removal of oily components present on the surface of the magnesium alloy may be retarded or the metal material may be discolored. Therefore, it would be desirable that the first aqueous solution fall within the composition indicated in Table 1.

After the removal of oils, fats and impurities from the surface of the magnesium alloy using the first aqueous solution, water washing can be preformed (S210). The water washing enables complete removal of the first aqueous solution remaining unremoved on the surface of the magnesium alloy.

A portion of the strongly alkaline first aqueous solution remaining unremoved on the surface of the magnesium alloy can react with a strongly acidic solution to be used in the subsequent step to impede chemical reactions on the surface of the metal and to make the metal surface uneven. Therefore, the water washing can contribute to an improvement in the efficiency of the subsequent step.

After completion of the degreasing, a second aqueous solution having the composition indicated in Table 2 is prepared to remove the magnesium oxide coating formed on the metal surface (S220).

TABLE 2 pH range adjusted by Treat- Treat- Composition water ment ment HNO₃ H₃PO₄ Additive dilution temp. time 600-800 g/L 200-400 g/L 5-10 g/L 1.0-2.0 35-50° C. 0.5-2 min

The additive contained in the second aqueous solution is a wetting agent. As explained in the first aqueous solution, the wetting agent may be a surfactant for allowing the second aqueous solution to permeate the metal.

Etching of the surface of the magnesium alloy with the second aqueous solution enables removal of the oxide coating formed on the metal surface at an appropriate rate while preventing the discoloration of the metal.

After completion of the etching, water washing can be performed (S220). As explained previously, the water washing can prevent a portion of the second aqueous solution remaining unremoved on the metal surface after removal of the oxide coating from causing damage to the appearance of the metal.

After completion of the etching and the water washing, a third aqueous solution having the composition indicated in Table 3 is prepared to remove smut present on the surface of the magnesium alloy (S230).

TABLE 3 Composition Treatment Treatment KOH Additive Temp. Time 50-200 g/L 5-10 g/L 35-50° C. 0.5-2 min

The third aqueous solution is used to remove the smut (desmutting) from the metal surface. The additive contained in the third aqueous solution may be a leveling agent that makes the metal surface flat.

It would be desirable to adjust the composition of the third aqueous solution as indicated in Table 3. If the third aqueous solution contains KOH in an amount smaller than the lower limit, only a portion of the metal surface may be desmutted. Meanwhile, if the third aqueous solution contains KOH in an amount larger than the upper limit, a hydroxide coating may be formed on the metal surface. Therefore, it would be desirable that the third aqueous solution falls within the composition indicated in Table 3.

After removal of the smut from the surface of the magnesium alloy, water washing is performed on the magnesium alloy (S230). Thereafter, a fourth aqueous solution having the composition indicated in Table 3 is prepared to form a conversion coating on the outer surface of the magnesium alloy (S240).

TABLE 4 pH range adjusted by Treat- Composition water ment Treatment HNO₃ H₃PO₄ HF dilution Temp. Time 600-800 g/L 200-400 g/L 0.5-10 g/L 2.0-4.0 20-60° C. 10 sec-4 min

By the use of the fourth aqueous solution, a coating can be formed on the conversion coating (S250).

Thereafter, a salt spray test is conducted to evaluate the corrosion resistance of the metal and a test is conducted to evaluate the adhesion between the coating and the metal. The results are shown in Table 5. In Table 5, R.N. is a rating from 10 to 0 which represents a degree of corrosion in terms of a ratio between the effective area and the corroded area. The greater the R.N. value, the smaller the corroded area (i.e., less corrosion).

TABLE 5 Step Cross Cut Salt spray time R.N. 1 Coating after formation 100/100 24 hr 10.0 of conversion coating 36 hr 9.4

The inherent color of the surface of the magnesium alloy can be maintained unchanged and the damage to the metal surface can be minimized. The magnesium alloy can be used as a material for a variety of devices due to its improved corrosion resistance. The conversion coating can be formed in an environmentally friendly manner because a highly toxic conversion coating agent, such as chromate or chromic acid, is not used.

As is apparent from the foregoing, a conversion coating formed by the method according to the embodiment of the present invention can improve the corrosion resistance of the magnesium or magnesium alloy because of it is highly reactive with an acid and an alkali and shows strong activity.

In addition, the method enables the formation of a conversion coating on the metal surface in an environmentally friendly manner without the need to use of an environmentally harmful substance, such as chromic acid.

Furthermore, even after the treatment of the outer surface of magnesium or a magnesium alloy, the inherent color of the magnesium can be maintained unchanged.

Although the present invention has been described herein with reference to its preferred embodiments, these embodiments do not serve to limit the invention. Accordingly, those skilled in the art will appreciate that various modifications are possible without departing from the spirit of the present invention as disclosed in the accompanying claims.

The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent application, foreign patents, foreign patent application and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, application and publications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure. 

1. A method for forming a conversion coating on the outer surface of magnesium or a magnesium alloy, the method comprising: degreasing the outer surface of magnesium or a magnesium alloy, followed by water washing; etching the degreased magnesium or magnesium alloy to remove an oxide coating present on the outer surface thereof, followed by water washing; removing smut present on the outer surface of the magnesium or magnesium alloy, followed by water washing; forming a conversion coating on the desmutted outer surface of the magnesium or magnesium alloy, followed by water washing; and forming a coating on the conversion coating.
 2. The method of claim 1, wherein, in the step of degreasing and water washing, the degreasing is performed using a first aqueous solution comprising 30-60 g/L of potassium hydroxide (KOH), 150-200 g/L of sodium hydroxide (NaOH), 20-30 g/L of an amine oxide, 5-10 g/L of sodium gluconate (C₆H₁₁NaO₇) and a wetting agent.
 3. The method of claim 1, wherein, in the step of etching and water washing, the etching is performed using a second aqueous solution comprising 600-800 g/L of nitric acid (HNO₃), 200-400 g/L of phosphoric acid (H₃PO₄) and a wetting agent.
 4. The method of claim 3, wherein the second aqueous solution is diluted with distilled water to a pH of 1.0-2.0.
 5. The method of claim 1, wherein, in the step of removing smut and water washing, removing smut is performed by immersing the etched metal in a third aqueous solution comprising 50-200 g/L of potassium hydroxide (KOH) and 5-10 g/L of a wetting agent.
 6. The method of claim 1, wherein, in the step of forming a conversion coating and water washing, the conversion coating is formed using a fourth aqueous solution comprising 600-800 g/L of nitric acid (HNO₃), 200-400 g/L of phosphoric acid (H₃PO₄) and 0.5-10 g/L of hydrofluoric acid (HF).
 7. The method of claim 6, wherein the fourth aqueous solution is diluted with distilled water to a pH of 2.0-4.0. 